Nip pressure sensing system

ABSTRACT

A roll sensing system for measuring the pressure distribution and nip width in a nip roll press. The sensing system comprises a strip having sensors thereon, the strip being placed in a nip press, for sensing the pressure at several locations therealong. At one end of the strip lies electronics associated with the sensors. The electronics communicate with an optional multiplexer and a bidirectional transmitter for signal transmission to an external signal conditioner and an external computer. The computer determines pressure values and nip width values at various locations along the strip, and communicates with a display which provides a visual, graphical and/or numerical data to the operator. Optionally, a control system can be in communication with the transmitter or the computer to initiate crown corrections in response to pressure or nip width readings.

This application is a continuation of Ser. No. 09/325,938 filed Jun. 4,1999, now U.S. Pat. No. 6,205,369.

FIELD OF THE INVENTION

The present invention relates to a system for use in connection with apress-nip section of a papermaking or related machine such as those usedin plastics calendering or steel making, such system being capable ofdetermining pressure as well as nip width distribution between nippedrolls.

BACKGROUND OF THE INVENTION

In the process of papermaking, many stages are required to transformheadbox stock into paper. The initial stage is the deposition of theheadbox stock onto paper machine clothing or felt. Upon deposition, thewhite water forming a part of the stock, flows through the intersticesof the felt, leaving a mixture of water and fiber thereon. The felt thensupports the mixture, leading it through several dewatering stages suchthat only a fibrous web or matt is left thereon.

One of the stages of dewatering takes place in the nip press section ofthe papermaking process. In the nip press section, two or morecooperating rolls press the fibrous web as it travels on the feltbetween the rolls. The rolls, in exerting a great force on the felt,cause the web traveling thereon to become flattened, thereby achieving adamp fibrous matt. The damp matt is then led through several vacuum anddewatering stages.

The amount of pressure applied to the web during the nip press stage isimportant in achieving uniform sheet characteristics. Variations in nippressure can affect sheet moisture content and sheet properties.Excessive pressure can cause crushing of fibers as well as holes in theresulting paper product. Conventional methods addressing this problemhave been inadequate, and thus, this problem persists in the nip pressstage, often resulting in paper of poor quality, having uneven surfacecharacteristics.

Roll deflection, commonly due to sag or nip loading, is a source ofuneven pressure distribution. Rolls have been developed which monitorand alter the roll crown to compensate for such deflection. Such rollsusually have a floating shell which surrounds a stationary core.Underneath the floating shell are pressure regulators which detectpressure differentials and provide increased pressure to the floatingshell when necessary.

One such roll is described in U.S. Pat. No. 4,509,237. This roll hasposition sensors to determine the existence of an uneven disposition ofthe roll shell. The signals from the sensors activate support orpressure elements underneath the roll shell, thereby equalizing anyuneven positioning that may exist due to pressure variations. Thepressure elements comprise conventional hydrostatic support bearingswhich are supplied by a pressurized oil infeed line.

A similar roll is disclosed in U.S. Pat. No. 4,729,153. This controlleddeflection roll has sensors for regulating roll surface temperature in anarrow band across the roll face. Other controlled deflection rolls suchas the one described in U.S. Pat. No. 4,233,010 rely on the thermalexpansion properties of the roll material, to achieve proper rollflexure. Such deflection compensated rolls are often useful for varyingthe crown. Thus, such rolls can operate as effectively at a loading of100 pounds per inch as at 500 pounds per inch, whereas rolls withoutsuch capabilities can only operate correctly at a single specificloading.

Notwithstanding the problem of roll deflection, the problem of unevenloading across the roll length, and in the cross machine direction,persists since pressure is often unevenly applied along the roll. Forexample, if roll loading in a roll is set to 200 pounds per inch, it mayactually be 300 pounds per inch at the edges and 100 pounds per inch atthe center.

Methods have been used to uncover discrepancies in applied pressure. Onesuch method requires stopping the roll and placing a long piece ofcarbon paper, foil, or impressionable film in the nip, which is known astaking a nip impression. However, one must load the rolls carefully toensure that both sides, that being front and back, are loaded evenly.The pressure in the nip transfers a carbon impression, deforms the foil,or ruptures ink containing capsules in the film, indicating the width ofcontact. These methods for taking a nip impression are not reusable asthey determine only a single event such as the highest pressure orcontact width.

One of the major difficulties in using the nip impression procedure, isthat of evenly loading the rolls from front to back. The goal of theprocedure is to measure and record the final stable loading along thelength of the rolls after the initial loading. Often during the initialloading, however, one end will contact before the other end. Thus, thereare times when one end is heavily loaded while the other end is onlyslightly loaded. When this occurs, the nip impression shows the highlyloaded condition and not the final state, since the carbon paper, foils,and prescale films record the largest width and/or highest pressures.

Another method of determining the nip pressure profile is to use aprescale film that measures pressure. The film is fed into the nip afterthe rolls are loaded. Therefore, the film records the stable loadedcondition, rather than the worst consequence of the loading process.Such a process eliminates the loading difficulties associated with nipimpressions. Nonetheless, the prescale films must be interpreted using adensitometer. This process is cumbersome, time consuming, and generallyinefficient. Furthermore, the prescale films are not reusable. A newpiece of film must be fed into the nip after any corrective adjustmentsare made. Lastly, the prescale films are temperature and moisturedependant, thus leading to inaccurate and unreliable results.

SUMMARY OF THE INVENTION

It is an object of the invention to measure pressure distribution alongthe length of a roll in a nip press.

It is another object of the invention to measure pressure distributionin the cross machine direction in a nip press.

It is yet another object of the invention to measure nip widths.

It is still another object of the invention to reuse the sensing systemat multiple nip press locations to measure pressure distribution and nipwidths on different length rolls.

It is yet another object of the invention to adjust the crown inresponse to irregular pressure distributions.

It is yet another object of the invention to adjust the crown inresponse to irregular nip widths.

It is yet another object of the invention to adjust the journal forcesor applied loads in response to irregular pressure distributions.

It is yet another object of the invention to adjust the journal forcesor applied loads in response to irregular nip width distributions.

It is still another object of the invention to provide a method ofdetermining the pressure in a nip press.

It is still another object of the invention to provide a method ofdetermining the nip width in a nip press.

These and other objects of the invention are achieved by a roll sensingsystem for measuring the pressure distribution and nip width in a nip.The sensing system comprises a strip having sensors thereon, the stripbeing placed between rolls in a nip press, for sensing the pressureand/or nip width at several locations along the roll. Electronicsassociated with the sensors communicate with an optional multiplexer anda bidirectional transmitter for signal transmission to an externalsignal conditioner and an external computer. The computer determinespressure values and nip width values at various locations along thestrip, and communicates such values to a display which providesgraphical and/or numerical data, visually to the operator. Optionally, acontrol system can be in communication with the transmitter, or thecomputer to initiate crown corrections in response to pressure or nipwidth readings.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below withreference to the following figures wherein like numerals represent likeparts:

FIG. 1: shows one embodiment of the sensing system of the instantinvention as it is applied on a roll in a press.

FIG. 2: shows a top view of a sensing strip used in the system accordingto the instant invention.

FIG. 3: shows a top view of an alternative configuration for the nipwidth sensors according to the instant invention.

FIG. 4A: shows a top view of another alternative configuration accordingto the instant invention wherein pressure sensitive lines on the sensorsare used to determine nip width.

FIG. 4B: shows a top view of a variation of the configuration of FIG. 4Awherein the pressure sensitive lines of the sensor are connect inseries.

FIG. 4C: shows a top view of still another configuration accordingwherein the sensor is inserted into the nip with the pressure sensitivelines arranged at an angle with respect to the roll length.

FIG. 4D: shows a top view of a variation of the configuration of FIG.4C.

FIG. 5A: shows a graphical display of the pressure measured at locationsalong the sensing strip in a nip press.

FIG. 5B: shows the sensing strip on a roll in the nip press,corresponding to the graphical display of FIG. 5A.

FIG. 6A: shows a graphical display of the nip width distributionmeasured in a nip press.

FIG. 6B: shows the sensing strip in a nip press corresponding to thegraphical display of FIG. 6A.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a preferred embodiment of the sensing system 1 of theinstant invention as it is applied to sense the pressure exerted byrolls 5, 6 in a nip press. In the nip press section of a papermakingmachine, rolls 5 and 6 rotatingly squeeze a fibrous web which is carriedon the felt 8 disposed therebetween. In order for the rolls 5, 6 toprovide uniform pressure to a fibrous web, they must be evenly loadedand the width of contact between the rolls, i.e. the nip width, shouldbe within a predetermined range.

The sensing system 1 comprises a strip 2, preferably an elongated membermade of a thin film of material. Sensors 4 are fixed to the strip forsensing pressure/force and/or nip width.

The strip 2 having sensors 4 thereon is shown for discussion purposes asnot contacting the felt 8 and roll 6. During system operation, however,the strip 2 must lie in the nip, between roll 5 and the felt 8 ordirectly between rolls 5 and 6. Placement of the strip 2 within the nipmay be achieved by removably attaching the strip to roll 5, as shown,and then rotating roll 5 to properly position the strip. Alternatively,the strip may be may be placed directly between rolls 5 and 6 and rolledinto the nip by rotating the rolls. One could also, open the nip formedby rolls 5, 6, place the strip between the rolls, and then close thenip.

The strip 2 having sensors 4 thereon, is preferably rolled into a coil15 for storage and unrolled during use. The sensors 4 are spaced on thestrip so that if a large number of sensors is required to effectivelydetermine pressure distribution or nip width, such as in the case of along roll, the operator can continue to utilize the strip. Thus, thesensing system can be used on any length roll, eliminating the need fordifferent length sensing systems for different rolls and/or mills. Also,several strips of sensors may be pieced end-to-end to span the length ofvery long rolls.

Although many types of sensors known to those skilled in the art wouldserve the desired function, the sensors 4 preferably comprise resistive,piezoelectric, piezoresistive, strain gage or fiber optic materials.Also, the sensors could be equipped with temperature measuring sensorsto aid in temperature compensation if needed.

In communication with the sensors 4 are lead wires 7, as shown in FIG.2, and associated electronics 10. The electronics 10 connected to thesensors 4 via lead 7, aid in converting the sensor signal to a pressuresignal, by amplifying the signal or eliminating external interference.The type of sensor used, however, determines the nature of theassociated electronics 10. For example, if piezoelectric orpiezoresistive sensors are used, the electronics 10 will comprise chargeamplifiers. Alternatively, if strain gage sensors are used, theelectronics 10 will comprise wheatstone bridges. If fiber opticmaterials are used, the electronics will comprise an optical phasemodulator.

The electronics 10 are in communication with a multiplexer 12 which isaccessed by a bidirectional transmitter 14. The multiplexer 12 cyclesthrough the sensors 4 to obtain pressure signals from sensor locationsalong the strip 2, and thus along the roll 5, in the nip press. Thebidirectional transmitter 14, transmits the signals from the multiplexer12 to a signal conditioner 16 which in turn, delivers conditionedsignals representing the pressure sensed, to the computer 18.

The sensors and associated electronics are preferably connected directlyto the computer via wire cable. Nonetheless, the signals may be sent viatelemetry or through slip rings. A preferred telemetry transmitter ismanufactured by MICROSTRAIN of Burlington, Vt. This telemetrytransmitter has a single channel FM bridge transmitter that can beswitched on and off remotely, to conserve power. An alternativetransmitter is manufactured by PHYSICAL MEASUREMENT DEVICES ofMelbourne, Fla. Model PAM-15 incorporates 15 channels over one radiolink.

The computer 18 has a microprocessor having the ability to access themultiplexer 12 at predetermined or requested times to obtainpressure-related or width-related data. Requested transmissions areachieved by operator input through the keyboard 19 of the computer. Oncethe computer 18 has indicated to the multiplexer 12 which channels toread, the computer 18 receives the signals from the sensors 4 associatedwith the channels or leads 7, selectively accessed by the multiplexer.

Such signals are delivered to the microprocessor which runs a softwareprogram to compute a pressure value or nip width value. Preferably,these values are then transmitted to a display 20 which providesnumerical or graphical cross machine pressure profiles and/or nip widthprofiles.

The computer 18 can further provide averages of the pressure or nipwidth values, as well as initiate a correction signal to an optionalcontrol system 22. In addition, the computer 18 can determine nip widthsindirectly from the pressure sensed, through analysis software such as“NipCalc”, produced by STOWE WOODWARD of Southborough, Mass. Thesoftware takes the pressure signals and provides output data relating tonip width, which is useful in making crown corrections. This can also beaccomplished through empirical relationships such as the ones used torelate nip width to line load, or through experimentally obtainedgraphs. Alternatively, nip widths can be sensed directly through the useof pressure sensitive material, to be discussed further with respect toFIGS. 3-4.

The control system 22 can be connected to the computer 18 or the signalconditioner 16 to correct any sensed pressure irregularities, byincreasing or decreasing the force being applied by the roll, or byincreasing or decreasing the degree of contact between the rolls 5, 6.The control system 22 has an internal computer 26 for receiving userinputs in response to interpretation of pressure sensed, or forreceiving direct pressure readings from the signal conditioner. Thecontrol system's computer 26, upon receipt of such signals initiatescorrective measures to adjust the force being applied by the roll 2.

Turning now to FIG. 2, the strip 2 preferably has sensors 4 disposedthereon at spaced locations. The spacing of the sensors is in accordancewith the usual practice for crown correction measurements. Although thesensors 4 are shown uniformly and linearly across the roll 2, thisconfiguration is not essential, as the placement of the sensors on theroll can appear in other configurations as well. Leads 7 emanate fromeach of the sensors, and travel to the associated electronics 10, toprovide signals representative of the pressure sensed to the multiplexer12. As discussed above, these signals are ultimately passed through thebidirectional transmitter 14 where they can be selectively accessed bythe computer 18 through the signal conditioner 16.

Alternative configurations for the sensors 4 on the sensing strip 2 areillustrated in FIGS. 3-4D. These configurations are desirable for thedirect determination of nip widths. FIG. 3 shows an alternative nipwidth sensor approach wherein the each sensor along the strip 2 is widerthan the largest expected nip width 21. The sensor signal changes inresponse to the area under pressure. Thus, the nip width 21 is relatedto the sensor response and the length of the sensor.

FIGS. 4A-4D illustrate alternative configurations wherein nip width isdetermined based on a change in the signal(s) from the pressuresensitive lines 17 in the sensors. Each pressure sensitive line 17 inthe sensor is connected to a lead line 7 through which a signalrepresentative of the pressure applied to the line: may be monitored.For the nip width sensor shown in FIG. 4A, the nip width 21 may bemeasured by counting the number of lines 17 in each sensor that providea signal indicating that they are under pressure. If the lines areequally spaced, then the nip width is calculated by the line spacingmultiplied by one less than the number of lines under pressure:

NIP WIDTH=δ [N−1]

where δ=the spacing between the lines,

N=number of lines contacted by the rolls.

The nip width sensitivity of this configuration is related to the spacebetween lines. For example, as the amount of space between the lines 17is decreased the accuracy of the nip width measurement improves.

An alternative to this embodiment is shown in FIG. 4B, wherein thepressure sensitive lines of the sensor 4 are joined in a seriesconfiguration. Here, the nip width 21 at the sensor 4 is related to thesignal measured at lead line 7 which is represents the pressure sensedalong the entirety of the series connected pressure sensitive line 17.

Yet another alternative sensor configuration is illustrated in FIG. 4C.Here, as in the configuration of FIG. 4A, the nip width 21 is measuredby counting the numbers of lines under pressure, but the nip widthsensitivity is dependant upon the offset spacing between the pressuresensitive lines. Thus, the nip width is estimated as:

NIP WIDTH=δ [(N)−w/δ]

where δ=the offset spacing between pressure sensitive lines,

N=number of lines contacted by rolls,

w=width of strip.

Another variation is shown in FIG. 4D. Here, the lines may also be at anangle with respect to the roll length. Nip width sensitivity in thiscase depends upon the offset spacing as well as the angle at which thelines intersect the roll axis.

Turning now to FIG. 5A, a graphical representation is shown of thepressure sensed along the length of the roll depicted in FIG. 5B interms of sensor location on the strip, set forth on the x-axis, versuspressure sensed, set forth on the y-axis. Referring to FIG. 5B, thepressure sensed by the sensor 4A at the front of the strip 2 representsthe pressure sensed at the front of the roll 5. Similarly, the pressuresensed by the sensor 4B on the strip represents the pressure at the endof the roll 5. The sensor 4B may, however, not be the last sensor on thestrip 2 due to the possibility of having an unused, coiled portion ofthe strip.

FIG. 6A provides a graphical representation of the nip widthdistribution for the rolls 5, 6 of FIG. 6B. This representation may beachieved using any of the strip configurations illustrated in FIGS.3-4D. As can be seen, the ends 27, 28 of the rolls are loaded moveheavily than the center and the corresponding nip widths are greater onthe ends. This loading distribution is commonly called “undercrowned”,indicating that the crown is too small for the journal loading. Auniform nip width distribution/pressure profile may be achieved byincreasing the crown or by decreasing the journal loads.

The general operation of the invention is as follows. One may approachthe test in many ways. One way would be to unload the rolls in contact.The sensor strip 2 is placed between the two rolls, leaving the unusedportion in a coiled configuration at the end of the roll 5. Roll 6 isthen loaded against roll 5, which has the strip 2 thereon. After therolls are loaded to the prescribed journal forces, usually measured byair bag pressures, the sensor strip 2 readings are acquired, asdiscussed above.

Another approach to test the nip pressure profile would be to load therolls at the prescribed journal forces, and then feed the sensor strip 2through the nip. The placement of the strip 2 may be achieved through arobotic arm or other automated equipment currently available. Inaddition, the strip 2 could be attached lengthwise to one of the rolls,or could be carried by the felt or web. The sensor readings would beacquired as the sensor passes through the nip.

At a predetermined, or at an operator-requested time, the computer 10communicates with the bidirectional transmitter 14, which furthercommunicates with the multiplexer 12. The multiplexer 12 then cyclesthrough the sensors 4, obtaining signals through the associatedelectronics 10, which signals are indicative of the pressure beingsensed by the sensors 4. The multiplexer 12 then communicates with thetransmitter 14 to send the signals to the signal conditioner 16 fordelivery back to the computer 10 where the determination of the pressurevalues takes place.

In the same manner, the operator can retrieve signals relating to thenip width sensed. The computer 18 then causes a numeric or graphicaloutput to appear on the display 20, alerting the operator of thepressure distribution or nip width in the static nip press. Optionally,the computer 18 and/or transmitter 14 can communicate pressure-relatedor width-related signals to the control system 22. In response to suchsignals, the control system 22 can then initiate crown correction toremedy any irregularities in the pressure sensed.

The system of the instant invention provides the operator with theability to determine the pressure profile of a roll in one or more nipsso as to diagnose the presence of unevenly applied roll forces. Thevarious graphical representations enable the operator to immediatelydetermine the pressure being applied, the location on the strip, thusindicative of the location along the length of the rolls, and whether ornot it is abnormal. Additionally, the system of the instant inventionprovides for corrective measures to be initiated in response to suchunevenly applied forces.

While the invention has been particularly shown and described withreference to the aforementioned embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention. Thus, any modification of the shape, configuration andcomposition of the elements comprising the invention is within the scopeof the present invention. It is to be further understood that theinstant invention is by no means limited to the particular constructionsor procedures herein disclosed and/or shown in the drawings, but alsocomprises any modifications or equivalents within the scope of theclaims.

What is claimed is:
 1. A system for measuring nip widths between tworolls in a nip press at a plurality of locations along the lengths ofthe rolls while the rolls are in a static condition, said systemcomprising: a flexible, elongate strip adapted to be placed in said nippress; a plurality of separate, spaced apart sensors disposed along thelength of said strip for providing signals representative of the nipwidth between said two nip rolls at the plurality of locations; each ofsaid sensors having an effective sensing length greater than theexpected nip width whereby said sensor may be stationarily positionedsuch that said effective sensing length extends entirely across the nipwidth and being configured such that, when said sensor is stationarilypositioned between the two nip rolls with at least a portion of saidsensor positioned within the nip width, said sensor is operative torespond to pressure exerted thereon by the two nip rolls by providing asignal proportional to the area of said sensor under pressure; and acomputer for accessing said signals and calculating valuesrepresentative of said signals.
 2. The system according to claim 1, saidsystem further comprising a display, coupled to said computer, forproviding a visual representation of said values.
 3. The systemaccording to claim 1, wherein said computer is adapted to performmeasurements of nip width from said signals.
 4. The system according toclaim 1, wherein said computer is adapted to automatically requestmeasurements of nip width at predetermined times.
 5. The systemaccording to claim 1, said system further comprising a wirelesstransmitter for communicating said signals to said computer.
 6. Thesystem according to claim 1, said system further comprising a controlsystem in communication with said sensors for initiating correctivemeasures to said nip press.
 7. A method for measuring nip widths in anip press comprising first and second rolls at a plurality of locationsalong the lengths of the rolls while the rolls are in a staticcondition, said method comprising the steps of: providing a flexibleelongate strip and a plurality of sensors disposed along the length ofsaid strip for providing signals representative of nip width betweensaid first and second rolls, each of the sensors having an effectivesensing length greater than the expected nip width and being configuredsuch that, when the sensor is stationarily positioned between the twonip rolls with at least a portion of the sensor positioned within thenip width, the sensor is operative to respond to pressure exertedthereon by the two nip rolls by providing a signal proportional to thearea of the sensor under pressure; positioning the strip between the twonip rolls as they are stationary with at least a portion of the sensorpositioned within the nip width and such that the effective sensinglength extends entirely across the nip width; while the sensors arestationarily positioned with respect to the nip width and the effectivesensing length extends entirely across the nip width, generating signalswith the sensors which are proportional to the areas of the sensorsunder pressure; computing measurements from said signals with a computerin communication with said sensors; and displaying said measurements ona display coupled to said computer.
 8. A method according to claim 7,said method further comprising displaying locations along the stripwhere the nip width is being sensed.
 9. A method according to claim 7,said method further comprising displaying the nip width of said nippress.
 10. A method according to claim 7, wherein said positioning stepcomprises positioning said strip against said first roll and loadingsaid second roll against said first roll.